Paleoclimatology

From Snowball Earth to the Anthropocene

Colin P. Summerhayes

• Publisher: Wiley-Blackwell
• Year: 2020
• ISBN:
  9781119591504
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Meteorology & Climatology

Life on our planet depends upon having a climate that changes within narrow limits – not too hot for the oceans to boil away nor too cold for the planet to freeze over. Over the past billion years Earth's average temperature has stayed close to 14-15°C, oscillating between warm greenhouse states and cold icehouse states. We live with variation, but a variation with limits. Paleoclimatology is the science of understanding and explaining those variations, those limits, and the forces that control them. Without that understanding we will not be able to foresee future change accurately as our population grows. Our impact on the planet is now equal to a geological force, such that many geologists now see us as living in a new geological era – the Anthropocene.

Paleoclimatology describes Earth's passage through the greenhouse and icehouse worlds of the past 800 million years, including the glaciations of Snowball Earth in a world that was then free of land plants. It describes the operation of the Earth's thermostat, which keeps the planet fit for life, and its control by interactions between greenhouse gases, land plants, chemical weathering, continental motions, volcanic activity, orbital change and solar variability. It explains how we arrived at our current understanding of the climate system, by reviewing the contributions of scientists since the mid-1700s, showing how their ideas were modified as science progressed. And it includes reflections based on the author's involvement in palaeoclimatic research.

The book will transform debate and set the agenda for the next generation of thought about future climate change. It will be an invaluable course reference for undergraduate and postgraduate students in geology, climatology, oceanography and the history of science.

"A real tour-de-force! An outstanding summary not only of the science and what needs to be done, but also the challenges that are a consequence of psychological and cultural baggage that threatens not only the survival of our own species but the many others we are eliminating as well."

Peter Barrett

Emeritus Professor of Geology, Antarctic Research Centre, Victoria University of Wellington, New Zealand

"What a remarkable and wonderful synthesis... it will be a wonderful source of [paleoclimate] information and insights."

Christopher R. Scotese

Professor, Department of Earth and Planetary Sciences, NorthwesternUniversity, Evanston, IL, USA

1
Introduction

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LEARNING OUTCOMES

• Understand that palaeoclimatology is concerned with the development of Earth's climate as measured by using proxies for environmental variables.
• Know about some aspects of the history of palaeoclimatology and how it emerged as a field.
• Understand some of the questions that underpin studies of palaeo climate.
• Understand the relevance of palaeoclimate studies to an understanding of modern and possible future climate states.

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1.1 What Is Palaeoclimatology?

Information about the climate of the past is referred to as palaeoclimate data (American spelling drops the second ‘a’). It relies on the use of proxies of climate variables to estimate the past behaviour of Earth's ocean–atmosphere‐ice system. Whilst proxy measures have their drawbacks, the fact that they mutually support one another provides a measure of confidence in palaeoclimatology.

The study of past climates used to be the exclusive province of geologists. They would interpret past climate from the character of rocks – coals represented humid climates; polished three‐sided pebbles and cross‐bedded red‐stained sands represented deserts; grooved rocks indicated the passage of glaciers; corals indicated tropical conditions, and so on. Since the 1950s we have come to rely as well on geochemists using oxygen isotopes and the ratios of elements like magnesium to calcium (Mg/Ca) to tell us about past ocean temperatures. And in recent years we have come to realize that cores of ice contain detailed records of past climate change as well as bubbles of fossil air. Glaciologists have joined the ranks. Nowadays the study of our climate system is the province of Earth System Science [1], a topic that treats the Earth's surface as parts of an integrated system in which everything is connected, much as Alexander von Humboldt first suggested back in the early 1800s.

Geologists are fond of saying ‘the climate is always changing’. They are correct, but that ignores the all‐important question – why? What we really need to know is ‐ how has the climate been changing, at what rates, with what regional variability, and in response to what driving forces? With the answers to those questions we can establish with reasonable certainty what the natural variability of Earth's climate is, and determine how it is most likely to evolve as we pump more greenhouse gases into the atmosphere. This book attempts to answer those questions in a way that should be readily understandable to anyone with a basic scientific education. It describes a voyage of discovery by scientists obsessed with exposing the deepest secrets of our changing climate through time. I hope that readers will find the tale as fascinating as I found the research that went into it.

The drive to understand climate change is an integral part of the basic human urge to understand our surroundings. As in all fields of science, the necessary knowledge to underpin that understanding accumulates gradually. At first we see dimly, but eventually the subject matter becomes clearer. The process is a journey through time in which each generation makes a contribution. Imagination and creativity play their parts. The road is punctuated by intellectual leaps. Exciting discoveries change its course from time to time. No one person could have discovered in his or her lifetime what we now know about the workings of the climate system. Thousands of scientists have added their pieces to the puzzle. Developing our present picture of how the climate system has worked through time has required contributions from an extraordinary range of different scientific disciplines from astronomy to zoology. The breadth of topics that must be understood for us to have a complete picture has made the journey slow, and still makes full understanding of climate change and global warming difficult to grasp for those not committed to serious investigation of a very wide ranging literature. The pace of advance is relentless, and for many it is difficult to keep up. And yet, as with most fields of scientific enquiry, there is still much to learn – mostly, these days, about progressively finer levels of detail. Uncertainties remain. We will never know everything. But we do know enough to make reasonably confident statements about what is happening now and what is likely to happen next. Looking back at the progress that has been made is like watching a time‐lapse film of the opening of a flower. Knowledge of the climate system unfolds through time until we find ourselves at the doorstep of the present day and looking at the future.

This book is the story of climate change as revealed by the geological record of the past 800 million years (800 Ma). It is a story of curiosity about how the world works, and ingenuity in tackling the almost unimaginably large challenge of understanding climate change. The task is complicated by the erratic nature of the geological record. Geology is like a book whose pages recount tales of Earth's history. Each copy has some pages missing. Fortunately the American, African, Asian, Australasian, and European editions all miss different pages. Combining them lets us assemble a good picture of how Earth's climate has changed through time. Year‐by‐year the picture becomes clearer as researchers develop new methods to probe its secrets.

1.2 What Can Palaeoclimatology Tell Us About Future Climate Change?

Whilst the story of Earth's climate evolution has a great deal to teach us, it is largely ignored in the ongoing debate on global warming. And yet the examination of the past to tell us what the future may hold is not a new idea. It was first articulated in 1795 by one of the fathers of geology, James Hutton. But it is not something the general public hears much about when it comes to understanding global warming. This book is a wake‐up call, introducing the reader to what the geological record tells us.

Increasing scrutiny of the palaeoclimate record over the past few decades helps us to explain why our present climate is the way it is. Most of the fluctuation from warm to cold climates and back through time takes place because of changes in the balance of Earth's interior processes. Changes over millions of years involve periods of excessive volcanic activity associated with the break up and drift of continents, which fills the air with CO₂ and keeps the climate warm, while periods of continental collision build mountains and encourage the chemical weathering of exposed terrain that sucks CO₂ out of the atmosphere, to keep the climate cool. Continental drift moves continents through climatic zones, sometimes leaving them in the tropics, sometimes at the poles. It also changes the locations of the ocean currents that transport heat and salt around the globe. Individual volcanic eruptions large enough to eject dust into the stratosphere provide short‐term change from time to time, while the equally erratic but more persistent volcanic activity of large igneous provinces involving the eruption of millions of cubic metres of lava over a period of a million years or so can change the climate for longer periods and did so enough at times to cause substantial biological extinctions.

External changes are important too. The Sun is the climate's main source of energy. Orbital variations in the Earth's path around the sun, combined with regular changes in the tilt of the Earth's axis, superimpose additional change on these millions of years‐long changes, through cycles lasting 400 000 to 20 000 years (400 to 20 Ka). Variations in the Sun's output superimpose yet another series of changes, with variability at millennial, centennial, and decadal scales, although their impact is surprisingly small. Examples of these include the 11‐year sunspot cycle and its occasional failure. Best known of those failures was the Maunder Minimum between 1645 and 1715 AD at the heart of the Little Ice Age. Large but rare meteorite impacts had similar, albeit temporary, and fortunately extremely rare, effects.

Internal oscillations within the ocean–atmosphere system, like El Niño events and the North Atlantic Oscillation, cause further changes at high frequencies but low amplitudes and are usually regional in scope. Whatever the climate may be at any one time, it is modified by internal processes like those oscillations, and by the behaviour of the atmosphere in redistributing heat and moisture rapidly, by the ocean in redistributing heat and salt slowly, and by the biota – for example through the ‘biological pump’ in which plankton take CO₂ out of surface water and transfer it to deep water and eventually to sediments when they die. These processes can make attribution of climate change difficult, as can the smearing of the annual record in deep water sediments by burrowing organisms.

In spite of the potential for considerable variation in our climate, close inspection shows that at any one time the climate is constrained within a well‐defined natural envelope of variability. Excursions beyond that natural envelope demand specific explanation. As we shall see, one such excursion is the warming of our climate since late in the last century.

Other processes are important too, notably weathering. In almost every churchyard you'll find gravestones so old that their inscriptions have disappeared. Over the years drop after drop of a mild acid has eaten away the stone from which many old gravestones were carved, obliterating the names of those long gone. We know that mild acid as rainwater, formed by the condensation of water vapour containing traces of atmospheric gases like carbon dioxide (CO₂) and sulphur dioxide (SO₂). It's the gases that make it acid. Rain eats rock by weathering.

Weathering is fundamental to climate change. Over time it moves mountains. Freezing and thawing cracks new mountain rocks apart. Roots penetrate cracks as plants grow. Rainwater penetrates surfaces, dissolving as it goes. In due course, scientists of a chemical bent realized not only that CO₂ contributed to weathering, but also that the CO₂ in the dissolved products of weathering reached the sea to form food for plankton, eventually reaching the seabed in the remains of dead organisms that would form the limestones and hydrocarbons of the future. One day, volcanoes would spew that CO₂ back into the atmosphere for the cycle to begin all over again.

The greenhouse gases are also important modifiers of climate, especially carbon dioxide (CO₂), which connects the climate to life and is part of the carbon cycle. The carbon cycle includes the actions of land plants, which extract CO₂ from the air by photosynthesis. Furthermore, as David Beerling reminds us, ‘roots and their symbiotic fungal associates secrete organic acids that attack mineral particles in soils to liberate nutrients needed for growth. Roots also anchor soils, slowing erosion and giving the minerals more time to be dissolved by rainwater. Meanwhile, above ground organic debris accumulates in a litter layer to form a continuous moist acidic environment that helps break down soil minerals’ [2] … Plants and their fungal partners dissolve rocks five times faster than normal, irrespective of their environment [2].

When plants die, they rot, returning their CO₂ to the air. Some are buried, preserving their carbon from that same fate, until heat from the Earth's interior turns them back into CO₂ and returns it to the air. This natural cycle has been in balance for millions of years. We have disturbed it by burning fossil carbon in the form of coal, oil, or gas.

There is a certain paradox to the story of plants. As they evolved, developing longer stems and broader leaves and more extensive roots, they extracted more of that CO₂ plant food from the air, causing its concentration to fall, and so cooling the climate. Disaster was averted as Earth's natural thermostat kicked in, with cooler conditions slowing the rates of chemical weathering and the supply of rain (a product of the concentration of water vapour, which diminishes in a cooling world). Ongoing volcanic activity built the CO₂ levels of the atmosphere back up again for the cycle to repeat. In time, new varieties of plant (mostly grasses, including such food crops as maize and sugar cane) evolved capable of extracting carbon dioxide more efficiently from the atmosphere in our incr...

Table of contents

• Cover
• Table of Contents
• Author Biography
• Acknowledgement
• 1 Introduction
• 2 The Great Cooling
• 3 Ice Age Cycles
• 4 Trace Gases Warm the Planet
• 5 Changing Geography Through Time
• 6 Mapping Past Climates
• 7 Into the Icehouse
• 8 Greenhouse Gas Theory Matures
• 9 Measuring and Modelling CO2 Back Through Time
• 10 The Pulse of the Earth
• 11 Numerical Climate Models and Case Histories
• 12 Solving the Ice Age Mystery – The Deep Ocean Solution
• 13 Solving the Ice Age Mystery – The Ice Core Tale
• 14 The Holocene Interglacial
• 15 The Late Holocene and the Anthropocene
• 16 Putting It All Together
• Appendix 1: Further Reading
• Appendix 2: List of Figure Sources and Attributions
• Index
• End User License Agreement